Approaches to direction finding typically use relative phase and amplitude differences between channels to estimate the angle of arrival (AoA) of the incoming wave. Two conventional approaches employed in direction finding systems use either two or three channels, with higher precision in the three channel system. Signal processing is performed to obtain the azimuth and elevation.
Conventional direction finding beamformers typically create a sum channel and either one or two difference (i.e., delta) channels. Three channel systems offer improved accuracy over existing two channel systems at the expense of higher power requirements and more processing due to the requisite increase in the number of channels by 50%. Thus, in conventional systems, designers must choose between higher performance and lower cost (as determined by size, weight, and power).
Angle-of-arrival receivers compare the relative phase and/or gain of two or more input channels in order to estimate the arrival angle from which energy is impinging the aperture. A conventional monopulse aperture has four coplanar quadrants with a beamforming network that creates a single sum channel and one or more difference channels. These four coplanar quadrants may be denoted A, B, C, and D, and these quadrants may represent 90° physical slices the aperture. The four quadrants 100 of a representative circular aperture are illustrated in FIG. 1. Counterclockwise from the lower left, A is the lower left quadrant, B is the lower right quadrant, C is the upper right quadrant, and D is the upper left quadrant. However, which quadrant of an actual aperture is denoted by which reference letter is relative.
A conventional three channel implementation forms Σ=A+B+C+D, Δaz=A−B−C+D, and Δel=A+B−C−D, where Σ is the sum channel, Δaz is the horizontal difference channel (left minus right), and Δel is the vertical difference channel (top minus bottom). Such a system is depicted in three channel system 200 of FIG. 2. A two channel system forms Σ=A+B+C+D and one of two symmetrical delta patterns, Δ1=A+jB−C−jD (clockwise) or Δ2=A−jB−C+jD (counterclockwise), where j is the imaginary number √{square root over (−1)}. An example is illustrated in two channel system 300 of FIG. 3. Note that in FIG. 3, the clockwise circular delta has been selected. Direction finding information is split equally between these two noise-independent circular delta channels. Thus, systems that use a single circular delta channel lose direction finding information. This impacts the radius of uncertainty of the estimated AoA by roughly √{square root over (2)}. Thus, conventional two channel beamformers, while saving system resources (e.g., power and processing), do not perform direction finding as well as three channel systems. Accordingly, an improved direction finding system that maintains the accuracy of a three channel system with the power and processing requirements, and therefore the cost, of a two channel system may be beneficial.